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Modeling Fe/N/C catalysts in monolayer graphene Xiaodong Yang, Yanping Zheng, Jing Yang, Wei Shi, Jin-Hui Zhong, Cankun Zhang, Xue Zhang, Yu-Hao Hong, Xinxing Peng, Zhi-You Zhou, and Shi-Gang Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02702 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 16, 2016
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Modeling Fe/N/C catalysts in monolayer graphene Xiao-Dong Yang,‡ Yanping Zheng,‡ Jing Yang, Wei Shi, Jin-Hui Zhong, Cankun Zhang, Xue Zhang, Yu-Hao Hong, Xin-Xing Peng, Zhi-You Zhou*, Shi-Gang Sun* Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, PR China ABSTRACT: Pyrolyzed Fe/N/C is one of the most promising non-precious metal catalysts for oxygen reduction reaction (ORR), which is supposed to boost the commercialization of proton exchange membrane fuel cells (PEMFC). However, the nature of active sites of Fe/N/C is not clear, and has long been debated. The challenges mainly come from highly heterogeneous structures formed during pyrolyzed process as well as no suitable surface probes. To elucidate the active sites, the most effective approach is building well-defined model catalysts as single-crystal planes in surface sciences. Herein, we designed single-atomic-layer Fe/N/C model catalyst based on monolayer graphene (FeN-MLG) to explore the active sites. The model catalyst was prepared by 950 oC heat treatment of graphene with controlled defects under FeCl3(g)/NH3 atmosphere. As-prepared model catalyst exhibits comparable ORR activity and SCN- suppressive effect with normal nanoparticle-like Fe/N/C catalysts, indicating that active sites are successfully created in the model catalyst. The effect of defect density, the layer number of graphene, and nitrogen species on the ORR activity had been investigated. The main content of nitrogen species on FeN-MLG is Nx-Fe, and quantitative correlation between Nx-Fe and ORR activity demonstrates that Nx-Fe species are the active site of Fe/N/C catalysts. The proposed model catalyst serves to simplify the catalyst structures and to simulate the topmost atomic layer of normal Fe/N/C, where ORR is catalyzed. This model system opens an opportunity to further understand the highly heterogeneous Fe/N/C catalysts. KEYWORDS. Oxygen reduction reaction, iron-based catalysts, monolayer graphene, model catalysts, active site. Introduction Today, platinum-based catalysts are exclusively used in the proton exchange membrane fuel cells (PEMFC) due to the sluggish kinetics of oxygen reduction reaction (ORR). For large scale commercialization of PEMFC, there is a considerable motivation to search for Earth-abundant materials to replace platinum-based catalysts. Iron and nitrogen doping carbon (Fe/N/C) catalyst is generally considered as one of the most promising candidates. Recently, significant progress of Fe/N/C catalysts has been made to the level comparable with platinum-based catalysts.1,2 However, the structure of the active sites has remained contentious even after 50 years of research. Various structures have been proposed as active sites in literatures, such as pyridinic N,3 iron encapsulated in graphene nanoshells,4,5 four-coordinated FeN4,1,6 Fe-N2+2 bridging in two graphene edge,7 and five-coordinate N-FeN2+2.8 Challenges in exploration of active sites are discussed as follows. Pyrolyzed Fe/N/C is inherently highly heterogeneous. Classical Fe/N/C has at least five types of nitrogen.9,10 And the environment of iron atom is more complicated,11 which is either linked (coordinated) to nitrogen or carbon, or incorporated as metal nanoparticles. Thus, recording the spectroscopic fingerprint of active sites is prevented. It becomes more difficult when considering the non-crystallographic ordering of the metal and nitrogen atoms. Complicated situation makes it difficult to determine the ORR activity of special species, like inorganic Fe. Fe/Fe3C nanocrystals were suggested to boost
the activity.4,5,12 While in the work of Jaouen13 and Kramm,14 they believed inorganic Fe species were ORR inactive, because equal ORR active was observed in the catalysts with or without inorganic Fe species. ORR is catalyzed on the topmost atomic layer. Inconsequently, regular analytical methods provide only bulk information of Fe/N/C catalysts, including Mössbauer spectra,7,13,14 X-ray absorption near-edge spectroscopy (XANES),7,13,14 time-of-flight secondary ion mass spectrometry (TOF-SIMS)15 and X-ray photoelectron spectrometry (XPS, near surface information).16,17 Without specific analysis of the catalyst surface structures where ORR happens, inexact or incorrect conclusions may be made. It is obvious that the exploration of surface species is much more important than bulk analysis. Efforts have been made to apply strong ligands as surface probes, such as CO (work in 193 K),18 halide ions and sulfur-containing species (e.g. SCN-, SO2, and H2S).10 Nevertheless, no suitable surface probes to date for Fe/N/C have yet been developed to discriminate moieties with a good resolution (e.g. iron in atomic dispersion and iron clusters). Hence, both of heterogeneous structures and lack of suitable characterization technologies limit the full understanding of Fe/N/C catalyst. Look back on the history of electrochemical catalysts, model catalyst provides a simplified system, which leads to an atomic-level understanding of reactions. The classic model catalysts, in the past, were the metal single crystal planes with well-defined surface atomic arrangement. Model system for non-metal catalysts has been developed
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recently in monolayer graphene. Graphene is a one-atom layer of sp2 carbon. The well-defined structure and interesting electronic/mechanical properties19-22 make graphene as a perfect model platform for fundamental electrochemistry researches. Dryfe group measured the electron transfer kinetics between redox mediators (Fe(CN)63– , Ru(NH3)63+, and IrCl62–) and carbon surface by modeling in monolayer and multi-layer graphene.23,24 Correlation between electrochemical activity and the defect density of carbon material was simulated in a modeling graphene with series of defects densities.25 Pyridinic nitrogen was proved to create ORR activity in a model catalyst with well-controlled N-doping species in highly oriented pyrolitic graphite (HOPG).26 In comparison with HOPG, graphene is a more ideal carbon material for preparing Fe/N/C model catalyst, because all of structure information provided by characterization technologies comes from the topmost atomic layer strictly, where ORR occurs. In addition, no metals were introduced into the HOPG-based model catalyst.26 It is well known that doping Fe can greatly the ORR activity greatly, especially in acidic medium. In this study, single atomic layer of Fe/N/C model catalyst was achieved by treating defective monolayer graphene (MLG) at 950 oC with FeCl3/NH3 sources. The model catalyst exhibits similar ORR behaviors with normal pyrolyzed Fe/N/C catalysts, including kinetic activities and SCN- suppressive effect. We further found that model catalysts in monolayer and multilayer graphene have a similar ORR activity. More importantly, Nx-Fe (>50%) is found to be the main content of the nitrogen species in model catalysts. Quantitative correlation between Nx-Fe and ORR activity is demonstrated. Results and Discussion
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temperatures and Fe/N sources were ready, graphene samples were moved back into sample-heating zone to achieve iron and nitrogen co-doping. Finally, Fe/N/C model catalyst in graphene were obtained. Also, MLG doped with nitrogen alone was prepared through the same procedure but only in NH3 environment. For abbreviation, defected MLG without heteroatom doping, with nitrogen doping alone and with iron/nitrogen co-doping were denoted as D-MLG, N-MLG and FeN-MLG, respectively. FeN-MLG_5min and FeN-MLG_10min were represented as iron/nitrogen co-doping samples achieved at 950 oC in 5 min and 10 min, respectively. And a sample denoted as FeN-KJ600 was made for comparison from KJ600 carbon black in the same synthesizing procedure. D-MLG
MLG
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C atom ‘graphitic’ N atom ‘pyridinic’ N atom ‘pyrrolic’ N atom Iron atom
Synthesis of Fe/N/C model catalysts in graphene Figure 1 shows schematic of building Fe/N/C model catalyst in MLG. The CVD monolayer graphene was transferred to a SiO2/Si substrate. Prior to heteroatom doping, defect in graphene was introduced by Ar+ irradiation. The irradiation treatment was carried out at a power of 10 W and a pressure of 500 mTorr with an Ar flow rate of 20 sccm. The exposure time was employed to control defect densities. Samples were then processed to heat-treating. As illustrated in figure 1, there are two heating zones in the tube furnace. One is for treating samples (graphene), and the other is for evaporating FeCl3 to supply Fe sources. Heat treatment was carried out in two steps. First step was for cleaning graphene sample. Graphene was heated up to 350 oC for 3 h under Ar atmosphere to remove any possible residue on the graphene surface. Thereafter, the sample was moved out of the heating zone by a magnetic rod. Second step of the heat treatment was for Fe/N doping. The sample-heating zone was heated from 350 oC to 950 oC, and Fe supplying zone was heated at 310 oC to sublimate FeCl3 solid. Both heating zones were setup to reach the target temperatures at the same time. NH3 gas was applied as nitrogen sources. When
Figure 1. Building Fe/N/C model catalyst in monolayer graphene. Prior to iron/nitrogen doping, defects in graphene + were introduced by Ar irradiation. A heat treatment proceo dure was performed at 950 C in FeCl3/NH3 to achieve Fe/Ndoping.
Single atomic layer of model catalyst As-prepared Fe/N/C model catalyst retained the single atomic layer structure of graphene, which was confirmed by atomic force microscopy (AFM), transmission electron microscopy (TEM) and selective area electron diffraction (SAED) as shown in Figure 2. Figure 2A demonstrates the AFM image of a folded FeN-MLG, as schematized at Figure 2B. From left to right, the line 1 was drawn from the bottom layer graphene to the upper layer graphene, and the line 2 was drawn from the upper layer graphene to SiO2/Si substrate. The corresponding height profile in Figure 2C reveals that one layer of FeN-MLG is about 0.5 nm. It is slightly higher than the expected valve (0.35 nm) of MLG, but still in the morphology of single atomic layer. To perform in TEM, MLG was transferred to a SiN TEM grid with φ 2.8 µm holes pattern, then doped with Fe/N.
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Figure 2D shows a bright-field TEM image of FeN-MLG. It is clean and uniform. Figure 2E shows SAED patterns, with the zone axis of [0001], selected at the holes. The inner hexagonal diffraction pattern was assigned to {1100}, and outer pattern was assigned to {1120}. Figure 2F presents the profile plots of the diffraction peak intensities along the white arrows in Figure 2E. Higher intensities of {1100} than that of {1120} reveals that FeN-MLG is in single atomic layer structure.27 Note that no nanoparticle species (such as Fe, Fe3C, FeNx) with dark contrast could be observed in the TEM image of FeN-MLG (Figure 2D). Supplying Fe source by sublimating FeCl3 salt during heat treatment may benefit the dispersion of Fe. As a result, the sample prepared by this approach is in absence of nanoparticles. In contrast, if the Fe source was provided by spin-coating diluted FeCl3 solution onto MLG, many particle-like species could be observed after heat treatment in ammonia atmosphere. And poor ORR activity was observed on such nanoparticle-containing samples. A
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Unlike nanoparticle catalysts, the graphene-based model catalyst just has single atomic layer, resulting in an advantage that the structure information provided by analytical technologies (e.g., XPS) is strictly limited on surface. Figure 3B shows the high resolution XPS of Fe 2p, which presents a direct evidence of successfully implanted iron into graphene in FeN-MLG. Higher iron content is observed in FeN-MLG_10min than that of FeNMLG_5min (2.2 at.% vs. 1.6 at.%). Almost the same C 1s spectra of five samples can be found in supporting information in Figure S1. The N 1s spectra are presented in Figure 3C. Nitrogen implanted in N-MLG and FeN-MLG is observed. Here, N 1s spectrum was deconvoluted into pyridinic-type nitrogen (B.E. 398.8 eV), metal-nitrogen (399.9 eV) and graphitic-type nitrogen (401.2 eV).30,31 With iron content increased, namely from N-MLG to FeN-MLG (either 5 min or 10 min), the content of pyridinic-N decreases, while metal-nitrogen increases.
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Figure 2. Characterization of Fe/N/C model catalyst in monolayer graphene (FeN-MLG). (A) AFM image of folded FeNMLG. The bright spot (B) Morphology schematic of folded FeN-MLG. (C) Height profile at the folded FeN-MLG marked by solid lines as 1 and 2, respectively, shows the thickness of FeN-MLG is about 0.5 nm. (D) A bright-field TEM image of the graphene. Scale bar: 200 nm. (E) Inset SAED patterns. (F) Profile plots of the diffraction peak intensities along the direction marked by white arrows in (E).
Raman spectroscopy is a powerful tool to characterize MLG.28 As shown in Figure 3A, five samples are presented, including MLG, D-MLG, N-MLG, FeN-MLG_5min and FeN-MLG_10min. The ratio of I2D to IG of MLG is around 2, means good quality of pristine MLG. D band is appeared in defective MLG, while 2D band is almost disappeared. After nitrogen or/and iron doping, Raman peaks blue-shifted due to the disorder and stress coming from heteroatom.29 Notice that defects are restored through
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ACS Catalysis
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Figure 3. Spectroscopic characterization of model catalysts, including iron/nitrogen co-doping MLG (FeN-MLG), nitrogen-doping MLG (N-MLG), defective MLG (D-MLG) and pristine monolayer graphene (MLG). (A) Raman spectra of FeN-MLG, N-MLG, D-MLG, and MLG, as D band and G band upshift with heteroatom doped. laser: 638 nm; spot size: ~500-nm; 100× objector. High resolution XPS of FeN-MLG, N-MLG, D-MLG, and MLG, Fe 2p in (B) and N 1s in (C). N 1s spectra were deconvoluted into pyridinic-type nitrogen (B.E. 398.8 eV), metal-nitrogen (399.9 eV) and graphitic-type nitrogen (401.2 eV).
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Similar ORR behavior was observed at SCN- ion suppression effect for FeN-MLG (Figure 4B) and for FeNKJ600 (Figure 4C). SCN- ion is believed to form a strong ligand with metal-center moieties, and then suppresses ORR.10 Loss of 45 % and 80 % ORR activity in FeN-MLG and FeN-KJ600, respectively, was demonstrated when 5 mM SCN- was added. After SCN- suppression, ORR current of FeN-MLG was even lower than that of N-MLG as evidenced by their polarization curves in Figure S4. The reason for higher current loss of FeN-KJ600 may be due to carbon nanoparticle has higher ratio of edge carbon, in which active sites are preferably speculated to form.32 The higher density of iron-center active sites in the FeN-KJ600 than FeN-MLG can also be used to explain the higher ORR activity of FeN-KJ600. In addition to ORR activity, the H2O2 yield or electron number is also an important parameter to evaluate ORR catalyst. Unfortunately, it is hard to transfer the FeN-MLG supported on Si substrate to a rotating ring-disk electrode (RRDE) for the measurement of electron numbers. Future work will be needed to measure this parameter, such as by designing a flow cell.
Practical Fe/N/C catalysts compose of carbon nanoparticles with multilayer graphene. Some active sites, such as encapsulated metal nanoparticles/clusters on carbon, are proposed in a structure with multilayer graphene. Is there activity difference between monolayer and multilayer graphene-based catalysts? To answer this question, we then extended our method to prepare model catalysts in multilayer graphene. The preparation is similar to above mentioned, just using a few layer graphene instead of monolayer graphene. Figure 5A shows Raman spectra of monolayer and multilayer pristine graphene. Broader peak width, less intensity and blue-shift of 2D band reveals the multilayer structure.33 Figure 5B shows the ORR polarization curves of model catalysts derived from graphene with different layer number. The kinetic currents of model catalysts in monolayer (FeN-MLG), 3~5 layer (FeN-3~5LG) and 6~8 layer (FeN-6~8LG) at 0.6 V are 4.67, 6.26 and 7.53 µA cm-2, respectively. Although the ORR activity is enhanced slightly with increasing the layer number of graphene, the difference is not significant (only increasing 60% from monolayer to 6~8 layers). This result indicates the same active moiety species are formed in monolayer and multilayer graphene.
j /µA.cm
Figure 4A shows dramatic increase of ORR activity from the pristine graphene to heteroatom doping graphene. Fe/N co-doping catalysts possess the highest ORR activity. The polarization curves were obtained by normalization of catalysts surface area and subtraction of background currents as shown in Figure S2. The Fe/N/C nanoparticle catalyst (FeN-KJ600) was synthesized in the same way as a comparison. Polarization curve of the FeNKJ600 was measured at rotating disk electrode (RDE), then normalized by Brunauer-Emmett-Teller (BET) surface area as well as corrected diffusion effect by KouteckyLevich equation as presented in Figure S3. The kinetic currents of FeN-MLG_10min and FeN-KJ600 at 0.7 V are in the same order of magnitude, being -0.133 µA cm-2 and -0.389 µA cm-2, respectively. Currents at 0.7 V were selected for comparison because it was in kinetic region for FeN-KJ600 (see supporting information in Figure S3). Note that the catalytic activity of FeN-MLG is much lower than commercial Pt/C yet (Figure S3C).
Figure 4. ORR behavior of FeN-MLG. (A) ORR polarization curves of FeN-MLG, N-MLG, D-MLG, MLG, and FeN-KJ600 were obtained in O2-saturated 0.1 M H2SO4 solution. Polarization curve of FeN-KJ600 was measured at rotating disk electrode at 900 rpm. (B) SCN ion suppressed ORR activity of FeN-MLG, which is similar to that of FeN-KJ600 (C).
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3~5LG MLG 1600 2600
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Figure 5. (A) Raman spectra of pristine monolayer, 3~5 layer and 6~8 layer graphene, normalized by G band. (B) Polarization curves of Fe/N/C model catalysts derived from monolayer, 3~5 layer and 6~8 layer graphene.
By further consideration of XPS and ORR results, the conclusion could be reached that the higher activity of FeN-MLG does not stem from the minor content of pyridinic-N. For example, the pyridinic-N content decreases from 2.4 at. % in N-MLG to 0.77 at. % in FeN-MLG_10min, while the activity is increased. To screen out direct correlation between the special moiety and ORR activity, catalysts with a series of nitrogen contents was designed by controlled defect densities on MLG. MLGs were exposed in Ar+ irradiation from 1 s to 25 s, and then processed Fe/N co-doping in a batch in 10 min. Figure 6A shows a series of Raman spectra of FeNMLGs with various defect densities. Raman spectra were
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normalized by G band. With the defect increase, D band increases first and then decreases, while D′ band increases and then merges with G band, and 2D band gradually disappears. A quantitative formula has been proposed to correlate the mean distance between defects in graphene (LD, nm) with the intensity ratio of ID/IG as shown in equation (1): 34,35 2 2 2 2 2 (r 2 − r 2 ) ID = CA 2A s 2 e−π rs / LD − e−π ( rA −rs )/ LD IG (rA − 2rs )
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To sum up, single atomic layer of Fe/N/C was successfully prepared as model catalysts. The good linear dependence of ORR activity on defect density and the amount of Nx-Fe moiety strongly indicates that active sites of Fe/N/C are the Nx-Fe formed at graphene defects. The layer number of graphene has little effect on the ORR activity. This study paves the route for exploring Fe/N/C in model catalysts designed from monolayer graphene.
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(>50 %). The ORR activity correlated on Nx-Fe and graphitic-N were plotted as shown in Figure 6C and Figure S9, respectively. A linear relationship between ORR activity and Nx-Fe is observed, which reveals that Nx-Fe moiety species is contributed to the ORR activity. However, in the case of graphitic-N, none of dependence relationship with activity can be found (Figure S9). Conclusions
Where rS (1 nm) and rA (3.1 nm) are the radii of the ‘structurally disordered’ area and the ‘activated’ area around the ion-induced defects, respectively. The factor CA is defined by the electron-phonon matrix elements. We used CA= 4.3 at the red line excitation (see in Figure S5).
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Figure 6. (A) Raman spectra of FeN-MLGs with a series of defects density, normalized by G band. (B) Correlation between ORR current at 0.6 V (j) and mean distance between defects (LD). (C) Correlation between ORR current at 0.6 V and percentage of Nx-Fe moiety species.
Figure 6B presents the dependence of ORR activity on the mean distance of defects. The larger LD means the lower density of defects. The currents of FeN-MLGs at 0.6 V increase with decreasing LD, namely increasing the defect density. The original polarization curves can be found in Figure S6. Notice that the shortest LD we made is 1.65 nm, it is difficult to create more defects due to the easily disconnected carbon matrix or irradiation out all of carbon atom. The activity would decrease because the increased resistance when highly heteroatom doping. Since Fe/N heteroatoms are supposed to occupy defective position, a linear relationship is observed between nitrogen contents and mean defects distance (Figure S7). High resolution N 1s spectra of XPS (Figure S8) show similar peak pattern in the series of FeN-MLGs with different defect densities. They were deconvoluted into pyridinictype nitrogen (B.E. 398.8 eV), metal-nitrogen (399.9 eV) and graphitic-type nitrogen. In all spectra, metalnitrogen, which is Nx-Fe in this case, is the main contents
Here, monolayer and multilayer graphene (3~5 layer graphene), bought from Xiamen G-CVD Material Technology Co., Ltd, was grown on a 25 µm-thick copper foil (Alfa Aesar, item No. 13382) by chemical vapor deposition (CVD) in a flow-type low pressure reactor. For transfer, a polymethyl methacrylate (PMMA) solution was spin coated onto the topside of the sample at 5000 rpm. Then the PMMA coat was kept at room temperature for 2 h to dry. Typically, graphene will grow on both sides of Cu foil and the backside graphene will hinder the Cu etch process. The backside graphene was removed by oxygen plasma etching for 3 min at a power of 100 W with a pressure of 315 mTorr. The Cu was etched in a potassium persulfate solution (15 mg/500 mL) for 24 h, and then further etched in a H2O2/HCl solution (ratio of H2O: H2O2: HCl is 20:1:1) for another 24 h. After cleaning in deionized water for 6 times, graphene with PMMA film was transferred to SiO2 (300 nm)/Si substrate and PMMA was removed by keeping it in chloroform overnight36. A 6~8 layer graphene film was prepared by the transfer of a PMMA-coated 3~5 layer graphene film onto a 3~5 layer graphene-coated Cu foil. And then it is continued by Cu etching and PMMA removing.37 X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) measurements were carried out on an Omicron Sphera II hemispherical electron energy analyzer (Monochromatic Al Kα with 1486.6 eV operating at 15 kV and 300 W). The base pressure of the systems was 5.0×10-9 mbar. XPS spectra were referenced to the carbon peak C1s at 284.6 eV. The N 1s spectra were deconvoluted into pyridinic-type N (398.8 eV), Metal-N (399.9 eV) and graphitic-type N (401.2 eV).30,31 The fitted curves were represented by 80 % Gaussian/ 20 % Lorentzian line shape after Shirley-type background subtraction. The full-width-at-half-maximum (FWHM) for each type of nitrogen component used for
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fitting was limited in the range of 1.1~1.50 eV, which was suggested by literatures.30 The final FWHM of 1.5 eV was selected because best-fitting results was obtained in this parameter. N 1s spectra of series of FeN-MLG_10min samples with various defect densities were fitted in the same parameter, including peak position, FWHM, Gaussian/Lorentzian line shape and asymmetry. ORR measurement The graphene-based model catalysts are cut into a rectangular shape. A copper wire was stuck to the graphene by conductive silver paint to contact with external circuit. The electrical contacts were then covered with epoxy. The exposed area of model catalyst was measured by a ruler. All the electrochemical measurements were performed in traditional three electrode cells. Model catalysts were used as the working electrode. A homemade reversible hydrogen electrode (RHE) was used as reference electrode. The electrolyte was 0.1 M H2SO4 and bubbled with high pure oxygen (99.999 %) at 40 sccm. The working electrode was subjected to potential cycling between 1.0 to 0.4 V (RHE) at a scan rate of 10 mV s-1. Unlike normal Fe/N/C catalysts, graphene samples had a low surface area, and the absolute current was less than 10 µA in this study. That is, the ORR process is fully controlled by instinct kinetic, not by O2 mass transfer in a wide range potential window. So, rotating disk electrode (RDE) is not needed during the ORR test. The geometric area was used to calculate the current density due to low roughness of graphene. Solution ohmic drop (i.e., iR drop) was compensated. The background capacitive current was recorded in the same potential range and scan rate, but in Arsaturated electrolyte. The current recorded in O2saturated solution was corrected for the background current to yield net ORR current (Figure S2). After the Arbackground (capacitive current) subtracted, the forward and backward of ORR polarization curves were averaged. Thus, polarization curves were obtained. For the FeN-KJ600, electrochemical test was carried out on a glassy carbon RDE. FeN-KJ600 ink was prepared in mixture solution (0.50 mL of water, 0.50 mL of ethanol and 0.05 mL of 5% Nafion) in concentration of 6 mg mL-1. The 25 μL of ink was pipetted onto the RDE, resulting in catalyst loading of 0.6 mg cm-2. A glassy carbon plate was used as counter electrode. Raman, AFM and TEM Raman spectra were carried out on an XploRA Raman instrument (Jobin Yvon-Horiba, France), which is equipped with 532 and 638 nm lasers. A 100× objective and 1200 lines mm-1 grating were used. About 10 spectra were measured for each sample, and a typical spectrum was selected. Raman spectra were normalized with G band. Atomic force microscopy (AFM) was performed on a NT-MDT system (NTEGRA Spectra) in tapping mode. Images with 10×10 μm2 area were obtained at a scan rate of 0.5 Hz using VIT_P tip.
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TEM images were obtained by TECNAI F-20 transmission electron microscope operating at 200 kV. The accelerating voltage used here (200 kV) is higher than the critical energy predicted for severe knock-on damage of graphene.38 Therefore, TEM images were quickly taken to avoid highly destroyed of graphene by electron beam. Prior to focusing process, several selective areas were labeled. Position was switched to the labeled area after focusing was done, and then diffraction pattern image was also quickly took.
AUTHOR INFORMATION Corresponding Author
[email protected] [email protected] Author Contributions ‡These authors contributed equally.
Funding Sources This work was supported by the National Basic Research Program of China (2015CB932303), Natural Science Foundation of China (21373175 and 21361140374), and Fundamental Research Funds for the Central Universities (20720150109).
ASSOCIATED CONTENT Supporting Information. Additional XPS, electrochemical characterization results, Ar adsorption/desorption isotherm of FeN-KJ600 and further discussion. This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT We thank Teng-Xiang Huang for help with the AFM measurements.
ABBREVIATIONS MLG, monolayer graphene; D-MLG, monolayer graphene with defects; N-MLG, nitrogen doping monolayer graphene; FeN-MLG, iron/nitrogen co-doping monolayer graphene.
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